It has been found useful in practice to record and play back television information on magnetic tape recorders in which the recording tape is looped at an angle around a stationary or rotatable guide drum while it is scanned by magnetic transducers mounted on a support wheel located within the guide drum. Due to the interaction of the relatively low transport speed of the magnetic tape and the relatively high rotational speed of the support wheel, the information is recorded on the magnetic tape as a series of parallel tracks which lie at an angle with respect to the edge of the magnetic tape. This type of recorder makes possible a very dense storage of the information and thus requires relatively small amounts of magnetic tape to store a given quantity of information.
In the evolution of the tape recording and playback apparatus for video tapes, it has been found useful to store the magnetic tape in a cassette which facilitates handling. In one design of a tape cassette, two coaxial reels hold the magnetic tape which passes from one reel to the other during the operation of recording or playback. A portion of the tape path lies within the cassette and is inclined with respect to the plane of the cassette. The overall difference in height which is traversed by the tape substantially corresponds to the height traversed by the tape in its loop around the transducer cylinder. Accordingly, the magnetic tape is removed for example, from the lower reel and passes on a parallel path to the plane of the cassette around a number of guide elements after which it loops the recording or playback mechanism so that its center line describes a part of a helix whose ascent on the cylinder is approximately equal to the width of the tape. Subsequently, the magnetic tape is guided around another set of guide elements and past a set of recording and playback heads which serve to record and play back a longitudinal track on the tape and this part of the path takes place in a plane which is parallel to the initial part of the path within the cassette. Subsequently, the tape is wound up on the upper cassette reel.
It follows from the above that, when the magnetic tape is stored in the cassette, it lies in a path which includes a portion that is inclined with respect to the plane of the reels. The angle of this internal inclination is determined by general requirements of construction of the cassette, for example by the reel diameter, the tape width or the separation between guide elements. On the other hand, when the cassette is placed in operational position within the apparatus and a loop of tape is removed from the cassette, it is desirable that the tape leave and enter the cassette substantially parallel to the plane of the cassette and that the inclined portion of the magnetic tape is substantially totally within the region of the recording and playback mechanism, i.e., in the tape loop around the transducer head.
In order to meet the requirement for long recording times, video recording tape is extemely thin so as to permit the storage of a large amount of information on a reel of a given size. As a consequence, video recording tapes are very sensitive to mishandling. Any longitudinal tension on the tape tends to impart a waviness to the tape edge which may make all the tape on a particular reel useless.
Attempts have been made in the past to develop mechanisms for loading and threading the magnetic tape from the cassette without the intervention of human hands. However, it has been found to be extremely difficult to remove the magnetic tape which is held in the cassette under a certain amount of tension without causing undesirable stresses to be imparted thereto, at least during certain transition stages. A particular difficulty has been the design of tape guiding elements which engage the tape immediately before and after its passage over the transducer and whose position must be fixed to very precise tolerances after the tape has been threaded. A further difficulty arises from the fact that the tape must be placed in a path whose angle of inclination is different from the angle with which it is stored within the cassette.
On the other hand, the inherent advantages available by the dense storage which is made possible by the looping of the transducer head and by the convenient packaging of magnetic tape in cassettes have led to the construction of magnetic recorders of relatively small dimension and weight which permit portable operation on battery power and enable the user to record news items and other transient information. In a portable tape recorder of this type, it is important to permit a relatively long operation on battery power so that any processes which require mechanical motion not directly related to the recording of information should be performed without the use of stored electrical energy.